Note: Descriptions are shown in the official language in which they were submitted.
CA 02217309 1997-11-13
The invention relates to a process for manufacturing
hot-worked elongated products, particularly bars or pipes,
from high-alloy or hypereutectoid steel.
High-alloy or hypereutectoid steels, especially
anti-friction bearing steels such as 100Cr6, form grain
boundary carbides and perlitic microstructural components when
cooled from high temperatures (1100 to 1250°C). These
formations impede mechanical workability and hardenability as
well as chipless deformation. A spheroidal cementite
microstructure suitable for further processing can be achieved
only after long annealing processes (spheroidal cementite
annealing) of 16 hours or more. Much thought has been given
to the question of how to shorten the duration of this soft
annealing whether the annealing can be replaced.
F. Mladen and E. Hornbogen studied the influence of
thermomechanical processing on the mechanical properties of
100Cr6 steel (Archiv Eisenhuettenwesen 49 (1978) No. 2, pp.
449 to 453). Austenitizing was carried out above the
temperature at which Fe3C completely dissolves which, given a
0.99 C w/o, is somewhat less than 1100°C. Hot rolling began
at 1100°C with simultaneous cooling to 720°C. Cooling from
720°C to ambient temperature was accomplished by water
quenching. The details of the deformation sequence are not
discussed in the article. The thermomechanically treated
microstructure displayed such a finely dispersed distribution
of carbides that the resolution limits of the optical
microscope were reached. The reason for this improved
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distribution was the increase in dislocation density and the
subgrain boundaries created by dislocations, which resulted in
new nucleation sites for the carbides.
A process for producing cylindrical rolled bodies
from steel with a 0.7 to 1.2 w/o is known from DE PS 2361330.
In this process, steel wire that has been hot-rolled at 1000°C
is rapidly cooled to a temperature that corresponds to its
lower pearlite range. The steel wire is then isothermally
transformed and brought to a hardness of 50 HRC by cold
drawing without intermediate annealing. The rapid cooling of
the wire and its subsequent isothermal transformation results
in a microstructure of fine-lamellar pearlite. This enables
the wire to be drawn, after being descaled and phosphatized,
without any intervening annealing.
The aim of the present invention is an especially
economical process for producing hot-worked elongated
products, especially bars or tubes, from high-alloy steel or
hypereutectoid steel, especially anti-friction bearing steel,
in which a microstructure is produced that is extremely well
suited, without prior soft annealing, such as to spheroidal
cementite annealing, for further chipless processing and final
heat treatment. A further aim is to produce a microstructure
that is also suitable, without prior soft annealing, for
further metal-cutting processing with a subsequent final heat
t rest ment .
The invention provides a process for producing a
hot-worked elongated element from one of a high alloy and a
hypereutectoidal steel, including the steps of: initially
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deforming a feed stock at a deformation temperature by feeding
said feedstock through a reducing mill at a predetermined
deformation temperature; producing a uniform temperature
distribution throughout a length and thickness of the deformed
feedstock after said step of initially deforming by controlled
heating or cooling to a preestablished temperature; repeating
the deformed feedstock to a temperature within one of a first
temperature range of 650 degrees C to Acl and a second
temperature range of Acl to Acma% deforming the repeated
feedstock to a final form by continuously rolling the repeated
feedstock in a multi-stand reducing mill for a total
deformation of ~ >_ 1.5 and an individual deformation of ~ >_
1.03 through each stand of the multi-stand reducing mill and
maintaining a temperature of said repeated feedstock within a
narrow range during said continuous rolling; and cooling the
finally formed feed stock to ambient temperature.
In a preferred embodiment Acl is 710°C and Acma is
880°C.
The coordinated process steps of the invention make
it possible to produce the desired microstructure, whereby, in
the case of the anti-friction bearing steel, a brinell
hardness less than or equal to 280 HB 30, preferably less than
250 HB30, is achieved. This microstructure also makes it
possible to feed hot-worked tubes directly to a processing
unit, without soft annealing. The manufacturing process of
the present invention is especially economical, because it
omits soft annealing and the transport and work steps
associated therewith. The hot-worked elongated products
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according to the invention can be processed by cold drawing,
cold pilger rolling, cold rolling or cross rolling.
In the drawings Figure 1 is a structure after a
prior art procedure including spheroidal cementite annealing
and Figure 2 is a structure after the procedure according to
the invention without an annealing step. The individual steps
that contribute to the success of the process according to the
invention are explained in what follows: The first process
step, which occurs after the initial deformation and before
repeating for subsequent continuous rolling, is equalizing a
temperature using a controlled heating or cooling to achieve
temperature equalization over the length and circumference of
the rolled material, which has various temperatures. The
equalization temperature is lower than the preset temperature
of the repeating furnace. The purpose of this measure is,
first of all, to precisely adjust the temperature of the
rolled material, and taking into account the opportunities to
regulate temperature in the repeating furnace. Secondly, the
measure is intended to achieve the most precise and
reproducible conditions possible for the temperature-dependent
measurement of wall thickness that takes place before the tube
enters the reducing mill. The measure chosen, i.e., heating
or cooling, depends on the thickness of the material to be
rolled. For example, in the case of a pipe push bench
arrangement, the temperatures of thick-walled tubes after the
initial deformations of piercing, elongation and striking are
above 700°C because the large mass retains heat. In such
cases, temperature equalization is achieved by controlled
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cooling to a preestablished equalization temperature in the
range between 650° and 700° C. In thin-walled tubes, which
cool very quickly, temperatures are frequently below 650°C.
In this case, temperature equalization is achieved by
controlled heating to a preestablished equalization
temperature in the aforementioned range of 650° to 700°C.
Actual repeating is carried out either to a
temperature below Acl (Critical temperature between pearlite
phase field and austenite phase field on heating) but above
650°C, or to a temperature above Acl but below Acma (critical
temperature between cementite-austenite phase field and
austenite phase field where a = the start of the carbide
dissolution regiony. It is necessary to take into account the
well-known fact that the Acl or Acma temperature depends
primarily on the carbon content of the material used and on
its deformation history. The former of the temperature ranges
mentioned above corresponds to the second phase region a +
Fe3C in the continuous TTT diagram, while the latter
temperature range corresponds to second phase region y + Fe3C.
A further measure in the proposed combination of
coordinated process steps relates to the final continuous
rolling process, preferably in a stretch reducing mill.
Unlike other rolling methods, this rapid continuous rolling
offers few opportunities for intervention. It is nonetheless
important for the proposed process that, first of all, a
minimum partial deformation, expressed as the stretching
1.03, be maintained in the reducing mill per each stand and
that, secondly, a minimum stretching degree be maintained for
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the total deformation ~ >_ 1.5. In special cases, the total
stretching can even be somewhat deeper, for instance, ~ >_ 1.4.
In addition, any temperature increase that occurs during
rolling due to loss work, or any temperature decrease that
results from excessive cooling, should be minimized. In all
cases, rolling must take place in the given two-phase region,
and the rolled material leaving the final stand must have a
temperature corresponding to that of the region in question.
This means that during the preferred rolling in the ~ + Fe3C
region, the temperature of the rolled material must not exceed
Acma~ compliance with this narrow temperature range is
achieved by cool means control; additional heat, in special
cases, from an external heating device; and variations in the
geometry of the rolls, roll speed and pass reduction. In
roller geometry, the pressed length is especially significant.
The process according to the invention is generally
applicable for all known tube-making processes that end in a
reducing mill with or without draught or in a sizing mill.
For example, the process can be used on a continuous tube
train, a plug train or an Assel mill. In particular, it is
suitable for the push bench method of producing seamless tubes
of anti-friction bearing steel. The feedstock for the process
according to the invention can be ingot cast material (forged
or rolled) or strand cast material (square or round), whereby
the strand cast material is deformed and annealed in a known
manner prior to rolling. Tests have shown that the process
can be used especially advantageously when the chemical
analysis of the known anti-friction bearing steel is modified.
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This relates, firstly, to the sulphur and phosphorous content
and, secondly, to the ratio of chromium to carbon. To avoid
possible melt-out at the grain boundaries when deformation
rates rise, the maximum sulphur and phosphorous contents
should each equal 0.005 w/o, taking into account the ratio of
manganese to sulphur due to the suppression of FeS. The melt-
out danger results from the high deformation temperatures
required during the initial deformation steps, when
deformation rates are such as to lead to corresponding
temperature increases. For this reason, the deformation rate
in the initial deformation steps is selected such that the
temperature in the interior of the rolled material, (the least
advantageous point) does not exceed 1170°C. In addition, low
S and P contents have an advantageous effect on any subsequent
chipless deformation.
With respect to secondary metallurgy, the declining
S and P contents are also advantageous in establishing a low
oxygen content in the melt, which leads to an improvement of
the oxidic purity.
The chromium-to-carbon ratio should be in the range
of 1.35 to 1.52, preferably 1.45. The carbon content then
equals 0.94 w/o for example, while the chromium content equals
roughly 1.36 w/o. Undesirable carbide banding can be
positively influenced via this ratio.
When anti-friction bearing steel is used, the cost
advantage that results from omitting soft annealing, which
otherwise would be necessary, can be further increased by
using a strand cast bar with no predeformation (in the cast
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state and without prior heat treatment (diffusion)) as the
f eedst ock .
Another improving measure relates to the cooling
step that follows the final deformation. After leaving the
rolling mill, the rolled material is cooled in resting air or
by an air shower to a temperature corresponding to a
microstructure located above the martensite point and below
the bainite nose in the TTT diagram. The deformed material is
held in this area isothermally for several hours. This method
has proved advantageous in the reduction of internal stresses.
This step can be carried out by placing the rolled material on
a cooling bed covered at a suitable point in a heat-insulating
manner, or by feeding the rolled material to a temperature
equalization furnace or tempering furnace.
To dispense with the hardening of individual
finished products after machining, it is further proposed that
the rolled material, after cooling, be heated to a temperature
in the range of 600° to 700°C, cooled and then tempered at a
temperature in the range of 180° to 210°C. After the heating
and tempering, the rolled material has a hardness
corresponding to the required final hardness of the finished
product.
The proposed new process technology for
manufacturing hot-worked elongated products, especially bars
or tubes, from anti-friction bearing steel has the following
advantages:
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a) The process eliminates investment expenditures for a
special annealing furnace and operating costs for long-
term spheroidal cementite annealing.
b) The process eliminates transport and work steps
(annealing, straightening) and thus reduces opportunities
for defects, resulting, in shorter operational run times,
and in more economical hot-worked products or cheaper
feedstocks for further deformation steps.
c) The process improves material exploitation by shortening
work sequences and attaining low decarburization depths
due to omission of oxidizing annealing. This leads to
small allowances and thus lower machining volumes and
allows customers to retain their gripping clamp
dimensions.
d) The process eliminates the requirement for straightening
due to the reduced deformation temperature, the rolled
material leaving the rolling mill has greater rigidity
and becomes sufficiently straight on the cooling bed.
Straightening can therefore be omitted, as a rule.
e) The process produces markedly fine grained
microstructure. During heat treatment, this leads to
higher and more homogeneous hardness and better
toughness. This has a positive effect on the later
useful life of the finished product, e.g., roller
bearings.
f) The process achieves a microstructure that can be
subjected, without additional heat treatment, to a cold
deformation process, e.g., cold drawing, cold pilger
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rolling, cold rolling or cross rolling. After stress-
relief annealing, cold drawn tubes have the same
properties as cold pilger rolled tubes.
g) The process saves money during melt production due to the
reduced S and P contents and the Cr and C contents set at
the lower limits. Minimizing carbide banding and
improving oxidic purity increases the useful properties
of the finished product.
The process according to the invention will be
described in greater detail in reference to an example. A
hot-worked tube with dimensions of 40.9 mm in external
diameter x 4.8 mm in wall thickness is to be produced from
100Cr6 steel on a tube push bench machine. From a strand cast
bar 220 mm in diameter and 11,000 mm in length, feedstock
ingots approximately 850 mm in length are cut. The feedstock
ingots of 100Cr6 steel are in the cast state, i.e., they have
not been heat-treated or predeformed. The cut ingots are
placed into a rotary hearth furnace and heated to
approximately 1140°C. After a total heating time of 150
minutes, the ingots are removed individually from the furnace
and, after pressurized water descaling, fed to a piercing
press. In the piercing press, the initial deformation into a
pierced piece takes place. In this example, the pierced piece
has the following dimensions:
Outer diameter 223 mm
Inner diameter 121 mm
Wall thickness 51 mm
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This deformation corresponds to a cross-sectional
reduction of 29.4 and stretching of ~, = 1.42. In this
example, the deformation rate equals 0.45 s'1 and influences
the optimal temperature window. After the piercing press,
another deformation occurs, namely elongation in a shoulder
mill. This deformation produces a shell with an outer
diameter of 192 mm, an inner diameter of 112 mm and a wall
thickness of 40 mm. The cross-sectional reduction is 30.7
and the stretching ~, = 1.44. During this deformation, high
temperatures arise on the inner surface during rolling.
Therefore, special care must be taken to ensure that the
temperature on the shell inner surface does not exceed 1170°C.
Otherwise, inner surface defects must be expected due to grain
boundary melt-out. Changes in roll speed and transport angle
can be used as control variables. The third deformation step
is striking on the push bench. A push bench billet with an
outer diameter of 122.8 mm, an inner diameter of 112 mm and a
wall thickness of 5.4 mm is produced as the selected final
size. After being pushed through a number of stands, the
billet from the bar is detached in a detaching mill in the
form of an internal die. The temperature of the billet
continues to drop until the extracting of the push bar and
reaches, in the described case, a level in the range of 650°
to 700°C. After extraction of the push bar, the billet plug
is created. According to the invention, the billet, before
entering the repeating device, is subjected to controlled
cooling to attain a uniform temperature distribution in the
range between 650°C and 700°C. In this case, a temperature of
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approximately 670°C is striven for. The billet is held for a
certain time in a heat-insulating buffer, so that heat can
flow from the areas of the billet with a higher temperature to
the areas with a lower temperature. The heat insulation
ensures that the total level of the billet temperature does
not fall below the preset target value. In this example, the
temperature of the repeating furnace is set such that a
temperature of roughly 740°C is achieved in the deformation
material. At this temperature, the billet runs into a stretch
reducing mill. This mill comprises a large number of three-
roll stands, which are arranged offset by 120° in a roll line.
For the selected example with the final dimensions of 40.9 x
4.8 mm, 29 stands are used. The partial deformation in the
base stands equals a cross-sectional reduction of between 7.1
and 8.1~. The total deformation equals 72.7 in keeping with
a st retching ~, of 3 . 66 . The deformat ion condit ions are
selected (i.e., the pass design and roll speed are chosen and
the cooling is adjusted) in such a way as to permit a slight
temperature increase to 760°C. This ensures that deformation
in the stretch reducing mill takes place completely in the two
phase region Y + Fe3C. After cooling, tubes of 100Cr6 steel
rolled in this manner have a microstructure that comes near to
the spheroidal cementite microstructure. The finely dispersed
microstructure consists of spheroidized cementite with slight
pearlite residues. The brinell hardness of the tube produced
in this fashion is below 250 HH30. The distribution of
hardness values is slight. The microstructure is finer than
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that achieved by standard spheroidal cementite annealing, as
can be seen by comparing Figures 1 and 2.
The tube produced according to the invention can be
further processed without additional heat treatment in a
chipless or metal-cutting fashion. This processing can
consist, for example, of cold drawing. By using one of the
- deliberate temperature control before entry into the
repeating furnace,
- reduced repeating furnace temperature, compared to the
usual method,
- rolling in the two-phase region,
- omission of spheroidal cementite annealing lasting over
16 hours,
a much thinner decarburized layer is obtained, compared to the
known prior art. The tube dimensions needed for machining can
therefore be reduced. Despite stress-relief annealing after
straightening, cold-drawn tubes with microstructure attainable
according to the invention have the same properties as cold-
pilgered tubes.
To make clear the difference between the new process
technology and the known prior art, products of the same final
size (40.9 mm outer diameter x 4.8 mm wall thickness) were
also rolled of 100Cr6 steel according to the usual method.
The hardness found in these tubes equalled 328 HB30 at a
repeating furnace setting of 1000°C. This hardness is so high
that spheroidal cementite annealing is required prior to
further processing.
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In producing thick-walled hot tubes, (for example,
60.3 x 8.0 mm), it is advantageous to control cooling based on
the TTT diagram such that an isothermal holding period is
introduced above the martensite point, but below the bainite
nose. The temperature range is preferably between 240° and
300°C. After a holding period of more than 3.5 hours in this
temperature range, cooling to ambient temperature can take
place.
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